Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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and a weak shoulder (III), and a similar pattern is exhibited by the spectrum of

CaO (edge beyond the instrumental limit). In the case of MgO, where the edge is

known to lie at even higher energies, only one band (II) and a weak shoulder (III)

are present below 52 000 cm

− 1

, although a spectral inﬂ ection near the limit suggests

the presence of another band (I) just above that position. The intensity of absorp-

tions labeled as II and III is a function of the speciﬁ c surface area of the powders

used, decreasing in the order MgO (210 m

− 1

) > CaO (110 m

− 1

) > SrO (6 m

− 1

For such a reason, in the spectrum of BaO ( < 1 m

− 1

) bands due to surface states

appear only as shoulders.

Absorptions related to surface states must be affected by interaction with adsor-

bates, and a typical example of the evolution in such a process of near UV bands

of AEO is reported in Figure 2.6 , where the spectral patterns obtained by contact-

ing MgO with water vapor are shown. The solid line is the spectrum of the sample

2.3 UV-Vis-NIR Absorption Spectroscopy 63

Figure 2.5 Diffuse reﬂ ectance spectra of

polycrystalline AEO outgassed at 1073 K.

Spectra of MgO, CaO and SrO were

recorded in the presence of ∼ 10

Pa of non -

reactive gas (e.g. O

) to quench possible

ﬂ uorescence emission. The R scale is

displaced vertically to avoid overlapping of

spectra. The dotted portions of the MgO and

CaO spectra at ν = 52 000 cm

− 1

(the vacuum

UV) are extrapolations. Reprinted from ref.

[46] with permission from Francis & Taylor

Ltd.

64 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

outgassed at 1073 K, while the other two curves are spectra recorded after contact

with increasing small doses of H

O vapor (with coverages lower than monolayer).

The main feature to be noticed is that the low - frequency shoulder is eroded pref-

erentially. In addition, as the chemical attack progresses, the band at 46 000 cm

− 1

is depleted and the low - energy tail of another band can be now observed, support-

ing the hypothesis of the presence of an absorption just above the instrumental

limit.

The increase of the sensitivity towards surface reactions along the band series

III > I I > I (when observed) accounts well for the involvement in such absorptions

of surface ions with coordination number decreasing in the same order III < I I <

I. As indicated above, AEOs have the rock salt structure, with both cations and

anions octahedrally six - coordinated in the bulk. Their surface morphology results

from the intersection of nano {001} planes, which leads to the exposure of ﬁ ve -

(5C), four - (4C) and three - (3C) coordinated ions of facelets, edges and corners

respectively. A general consensus has been reached that such sites are responsible

for the optical transitions resulting in bands I, II and III, respectively. On this

basis, the trend in the intensities of such bands for each AEO (Figure 2.5 ) can also

Figure 2.6 The effect of H

O vapor adsorption at 293 K on the

diffuse reﬂ ectance spectrum of polycrystalline MgO initially

outgassed at 1073 K: (a) original spectrum, (b) after

adsorption of a ﬁ rst sub - monolayer dose of H

O, (c) after

adsorption of a second dose. Reprinted from ref. [46] with

permission from Francis & Taylor Ltd.

be rationalized, with sites in corner positions being deﬁ nitely less abundant than

those on edges, which are in turn much less numerous that those on faces. In

addition, this assignment accounts well for the changes in relative intensity of the

three bands in dependence on the speciﬁ c surface area of the material also, the

sensitivity of the relative amount of surface sites to this parameter following

the scale 3C >> 4 C >> 5C. On such a basis, the relative intensity of near - UV bands

in the spectra of a series of powder samples of an AEO differing in surface mor-

phology can be coupled with reactivity data to elucidate the possible structure

sensitivity of a catalytic process occurring at its surface [47] .

Besides being a probe of the presence of sites in different coordination states

and of their different reactivity, near - UV excitonic bands of insulating oxides can

be further analyzed to obtain insights into the electronic features of surface sites

responsible for such transitions, and the reasons for the peculiar reactivity related

to a type of surface site/structure. To achieve this, it must be recalled that the main

model for the quantitative prediction of the energies of surface states of highly

ionic solids has been developed by Levine and Mark [48] , where the exciton gap

for surface ions, E

, is expressed as:

EAIV

=−+2

(2.6)

where A (the electron afﬁ nity of the anion) and I (the ionization energy of the

cation) are assumed to be the same as for ions in the bulk, and V

is the Madelung

potential for surface ions, which (neglecting the sign) is given by

Vzea

=α

(2.7)

where z is the charge of the ions, α

the surface Madelung constant and a the

anion – cation distance.

Equivalent relations can be derived for excitonic bulk transitions, by considering

the bulk Madelung potential. Independently of the type of material, which deter-

mines the values of A, I and a , the dependence of E

on α

should be similar, so a

plot of E

against 1/ a for isostructural ionic materials should be a family of curves

with no tendency to intersect. However, this is not the case when the values of the

excitonic absorptions of AEO are considered, including those due to bulk excitons

(Figure 2.7 ). The rationalization of such behavior has been provided by assuming

that A and I depend not only on the material but also on the co - ordinative state of

the ions. In chemical terms, this means that the ionicity of the surface has to vary

according to different coordination situations; in particular, ionicity decreases as

the ion coordination decreases. Departure from ideal ionicity is then least for sites

responsible for excitonic band I, for which a 5C surface conﬁ guration is proposed,

and greatest for those responsible for excitonic band III, that contains 3C ions.

Ions exposed at the surface in different co - ordinative states are then character-

ized by different electronic features, which, in the case of oxygen anions of an

AEO, result in an increase of their nucleophilic character so relevant to the appear-

ance of the strong basic reactivity typical of these oxides.

2.3 UV-Vis-NIR Absorption Spectroscopy 65

66 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

2.3.4

UV Absorption Bands of Semiconductor Oxides

Many oxides employed as catalysts/photocatalysts, supports for catalysts or, vice

versa, supported catalysts, such as SnO, ZnO, Fe

, TiO

, ZrO

, CeO

, V

and WO

, are semiconductors, and their absorption edge due to the inter-

band electron transition fall in the near UV - Vis range. As reported in Section 2.3 ,

in semiconductors excitons undergo ionization, so the optical behavior of these

solids is dominated by the fundamental adsorption edge, which, by using the

appropriate equation, can be used to determine the E

of the material.

Among the various possibilities, it is worth noting that in semiconductor oxides

where the fundamental absorption edge is due to allowed transitions between

indirect valleys (see Figure 2.4 B), such as TiO

[49, 50] , WO

[51] and MoO

[52] ,

the absorption of photons can occur by coupling with the absorption or emission

of a phonon.

Hence, two adsorption coefﬁ cients must be considered [53] , one ( α

) for the

process with phonon absorption:

αν ν

agpp

hAhEE EkT

()

=−+

()()

−

[]

−21

1exp

(2.8)

and a second one ( α

) for the process with phonon emission:

Figure 2.7 Energies and surface transition versus the

reciprocal of the anion - cation distance. Reprinted from ref.

[46] with permission from Francis & Taylor Ltd.

αν ν

egpp

hAhEE EkT

()

=−−

()

−−

()

[]

−21

1exp

(2.9)

where E

and E

are the energy and phonon energy, respectively, and the terms in

square brackets contain the dependence on the number of phonons of energy

Since both phonon and emission absorption are possible when h ν > E

+ E

, the

absorption coefﬁ cient is then

αν α ν α νhhh

()

(2.10)

which results in the type of dependence of α

1/2

on h ν displayed in Figure 2.8 A,

from which the value of E

can be easily obtained. It is worth drawing attention

to the fact that that α

( h ν ) and α

( h ν ) exhibit a different dependence on the tem-

perature also, as when T is very low (as T

in Figure 2.8 A), the phonon density is

very small (large denominator in Equation 2.11 ), and therefore α

( h ν ) is very small.

Turning to experimental data, a similar trend in the dependence of α on h ν has

been obtained by Serpone and coworkers [54] in analyzing the spectra of colloidal

TiO

particles (Figure 2.8 B).

For each type/family of semiconductor oxide the value of E

is related to signiﬁ -

cant dimensional, structural and/or functional properties, and so the features of

the optical absorption edge is a source of relevant insight into such materials.

As for the functional properties directly dependent on the optical absorption

edge, a special place is held by the photocatalytic behavior of titania [49, 50] . In

this respect, the determination of E

of TiO

materials by electronic spectroscopy

data is of great interest. This allows assessment of the effectiveness of treatment/

preparation methods intended to decrease the wide E

(3.2 eV for the anatase form,

usually the most active photocatalytically). This is of interest in the quest for visible

2.3 UV-Vis-NIR Absorption Spectroscopy 67

Figure 2.8 Plot of α

− 1

against E ( h ν ): panel (A) as from

equations and panel (B) for TiO

colloidal particles (three

samples: 2.1, 13.3 or 26.7 nm) suspended in an aqueous

medium (15 g L

− 1

). Panel (A): adapted from ref [26] ; panel (B):

American Chemical Society.

68 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

light - driven photocatalysts, since the UV light required for the parent oxides is

only ca. 5% of ground - level solar radiation (the most desirable irradiation source).

Signiﬁ cant results have been obtained in the development of the so - called “ second

generation ” TiO

photocatalysts [55] obtained by implantation of TMIs, resulting

in an actual red - shift of the transition edge (Figure 2.9 A), instead of the appearance

of new spectral components assignable to transitions involving localized states

(Figure 2.9 B). Band gap narrowing has been also obtained by using another physi-

cal method, radio - frequency magnetron sputtering, to prepare TiO

in the form of

thin ﬁ lms [55] . Another important possibility for narrowing the TiO

band gap is

offered by the preparation of TiO

materials doped with non - metal impurities, also

introduced by using more conventional preparation methods (e.g. sol – gel). In

recent years this area has been the object of intense research activity (e.g. N and

C doping, see ref. [56] ).

Turning to the dependence of E

on dimensional properties, particle size

( < 100 nm) can have an effect on the spectral properties of semiconductors when

it becomes comparable with the size of the exciton. This gives rise to the so - called

Q - size effect (explained in chemical valence terminology in ref. [57] ) which, with

constant bonding geometry, results in a blue - shift of the absorption edge [58] .

Equations relating the band gap shift ∆ E

(measured spectroscopically) to dimen-

sional parameters have been established [57] , and a relationship between the

particle size estimated in this way and that obtained by other methods, typically

TEM, was found (see, for example, ref. [30] ). This relationship notwithstanding,

in some cases other effects can be responsible for the adsorption edge blue - shift.

For instance, the spectroscopic behavior exhibited by TiO

particles with decreas-

Figure 2.9 DR UV - Vis spectra of TiO

doped

with Cr ions with different methods. Panel A:

(a) unimplanted pure TiO

, (b) – (d) TiO

implanted with increasing amounts of Cr

ions: 2.2, 6.6 and 13 × 1 0

− 7

mol Cr/g TiO

respectively. Open circles are the action

spectrum of the material (d) for the

photocatalytic decomposition reaction of NO.

Panel B: (a) undoped TiO

, (b ′ ) – (d ′ ) TiO

chemically doped with increasing amounts of

Cr ions: 1.6, 20, 100, 200 × 1 0

− 6

mol Cr/g

TiO

, respectively. Reprinted from ref. [11]

with permission from Elsevier.

ing particle size upon anaerobic illumination has been alternatively interpreted on

the basis of the Burstein – Moss effect [59] , which is a consequence of the fact that

the Fermi level of electrons in particles is a function of the irradiation intensity.

However, at spectrophotometric light intensities normally used to record absorp-

tion spectra, the Burstein – Moss effect should be of little consequence.

In the ﬁ eld of oxide catalysts, besides the case of the optical behavior of nano-

metric powders of TiO

(see ref. [54] and references therein), a typical manifesta-

tion of the Q - size effect is the blue - shift of the absorption edge exhibited by very

small three - dimensional particles formed by increasing the loading when semi-

conductor oxides (e.g. V

, MoO

, WO

, Fe

) dispersed on a support (SiO

TiO

, ZrO

, Al

) are prepared.

Higher levels of dispersion (usually obtained at lower loadings) result in the

formation of bidimensional patches and monodimensional ribbons of MO

(M =

metal) species, which can be recognized on the basis of a third feature of E

, that

is, its sensitivity to bonding geometry/structure of semiconductor species. A sys-

tematic experimental and theoretical study on polyoxometallates [60, 61] demon-

strates that the fundamental optical absorption strongly relates to the number of

nearest MO

polyhedral neighbors and the number of bonds between each of those

neighbors (corner - or edge - shared polyhedra). For instance, a larger fraction of

edge - sharing polyhedra relative to corner - sharing ones should result in a greater

molecular orbital overlap between such units, and thus a smaller E

because of

the more extensive sharing of electron density between polyhedra. Conversely, the

local symmetry around M

n +

centers in polymeric structures and the metal – oxygen

bond lengths were found to have less inﬂ uence. On this basis, and by considering

appropriate reference materials (another key point in the elucidation of structure/

optical feature relationships [60] ), the determination of E

values from the optical

absorption edges can trace the evolution of the structure of supported species as

a function of loading. Furthermore, the trend obtained can be combined with that

exhibited by some functional aspect of interest, to elucidate structure/function

relationships, as performed in the case of WO

/ZrO

(Figure 2.10 ) [62, 63] and

/ox (ox = SiO

, ZrO

, Al

) catalysts [64] .

The validity of the approach based on the evaluation of E

from spectroscopic

data disappears as the size of such MO

decreases towards the limit of isolated

species, as bands of energy are turned into discrete energy levels. Hence, the

optical behavior can be described in terms of localized charge - transfer transitions

involving molecular orbitals. This will be the subject of the next section.

2.3.5

Highly Dispersed Supported Oxo - Species and TMI

2.3.5.1 LMCT Transition Bands as Source of Structural Insight

Highly dispersed surface species, with the limiting form of single - site active

centers, play a primary role in a number of catalytic materials because of their

peculiar features in terms of activity and selectivity. Both oxo - species and transition

metal ions supported on oxides (or in zeotype materials) belonging to these types

2.3 UV-Vis-NIR Absorption Spectroscopy 69

70 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

Figure 2.10 Indirect absorption edge energies

of WO

- ZrO

samples at several oxidation

temperatures and W loadings. Two crystalline

W oxide materials (monoclinic WO

and

ammonium metatungstate) are shown for

reference. Dashed curve is a summary of o -

xylene isomerization rates per W atom (523 K,

0.66 kPa o - xylene, 100 kPa H

). Reprinted with

American Chemical Society.

of species can be effectively investigated through their ligand to metal charge -

transfer absorptions falling in the UV - Vis range. The main features related to the

nature of such transitions have been reported in Section 2.2 , while this part will

be devoted to examples showing how structural insights into supported species

can be provided by the analysis of the features of LMCT bands. Some information

arises from the sensitivity of LMCT to the number of ligands surrounding the

metal ions, resulting in a decrease of the transition energy as the coordination

number increases. A typical example is depicted in Figure 2.11 , dealing with the

evolution of the local structure of isolated V

oxo - complexes grafted on the inner

walls of a MCM - 48 silica support as a function of the hydration level [65] .

For each type of coordination number, the position of LMCT bands are sensitive

to the presence of M

n +

–

n +

linkages. This is found, for example, for species

ranging from supported chromates to dichromates and polychromates [66, 67] ,

and for isolated or polymeric VO

tetrahedra present in Na

and NH

respectively, which are considered as reference compounds for the interpretation

of the spectra of supported V

species [68] .

As a further aspect, it must be considered that in the context of a type of struc-

ture, differences in shape and positions of LMCT bands can monitor the occur-

rence of peculiar local geometries or distortions. Such spectral features can usually

be analyzed in more detail, with a consequent higher information output, in the

case of catalysts with active centers that are quite homogeneous in structure and

with simpler spectra, as more commonly occurs in the case of highly isolated

species. These are the conditions for the observation of spectral behavior that can

be rationalized in terms of differences in the bond angles connecting the metal

active sites to a crystalline or amorphous host framework, as in the case of Ti(IV)

dispersed in silicalite or at the surface of amorphous silica [69] . Furthermore, it

must be noted that changes in the local structure of supported species can occur

owing to the adsorption of probe molecules, as in the interaction of NH

with Fe

sites, again in silicalite [70] . As shown in Figure 2.12 , in the presence of NH

, dis-

tinct CT absorptions were present (solid line), corresponding to framework Fe

species in an almost perfect tetrahedral symmetry, as the charge balancing of

the negative charge on the zeolite framework can be made by NH

, while after

vacuum treatment the CT bands appeared more complex and broader (broken and

dotted lines) indicating the formation of Fe

species with reduced symmetry.

Finally, according to the empirical optical electronegativity theory, CT absorp-

tions are sensitive to factors affecting the electronegativity of the ﬁ rst coordination

shell surrounding the metal center, such as the composition of the second shell.

Such dependence can be useful in the elucidation of the types of links saturating

the coordinative demand of a metal center exposed on a surface, in terms of the

possible presence of OH groups, instead of only surface lattice oxygen atoms.

This has been the case of tripodal [(OH)Ti(OSi)

] versus tetrapodal [Ti(OSi)

]

Ti(IV) sites anchored to the inner walls of mesoporous MCM silicas [71] . On the

basis of ab initio calculations, the presence of OH groups in the coordination

sphere was expected to produce a CT sub - band at wavelengths longer than 220 nm,

in agreement with the experimental evidence provided by the comparison of the

2.3 UV-Vis-NIR Absorption Spectroscopy 71

Figure 2.11 Evolution of DR UV - Vis spectra as a function of

hydration time (under ambient conditions) for a VO

/MCM - 48

catalyst (V/Si = 0.05): (a) time 0, (b) 10 min, (c) 30 min,

(d) 2 h. Reprinted with permission from ref. [65] ; Copyright

1996 American Chemical Society.

72 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

experimental DR UV - Vis spectra of original Ti

–

MCMs with those of the materials

after silylation, which converted surface OH groups Ti

–

OH and Si

–

OH into

–

OSi(CH

)

and Si

–

OSi(CH

)

The latter actually appears to be characterized

by a decrease of adsorption intensity in the 220 – 240 nm region (Figure 2.13 ).

2.3.5.2 d – d Transition Bands as a Source of Structural Insight

When oxide catalysts contain TMI in d

≤ d

conﬁ guration, additional detailed

information on the structure of metal ion sites can be provided by the analysis of

the features of their d – d bands. As indicated in Section 2.1 , for such an analysis

a huge background of literature is available. Selected examples of the different

spectral features that can be considered for the derivation of structural and chemi-

cal insight for TMI sites in catalysts, or on the evolution of the TMI sites during

catalyst preparation, will be presented.

Figure 2.12 Effect of vacuum treatment and NH

adsorption

on the DR UV - Vis spectra of a Fe - silicalite catalyst (Fe =

1.71 wt%; calcined in air at 773 K): broken line, outgassed at

673 K; full line, after contact with 60 Torr NH

; dotted line:

outgassed at 673 K. Inset: spectral behavior related to d – d

transitions; related comments in the original paper. Reprinted

from ref. [70] , with permission from Elsevier.